Concrete Contraction Joints

We are often asked questions about those “lines on my floor” and whether or not we fill them in.  Those lines are actually joints.  There are two types of joints you could be seeing in your garage floor and they are called Contraction Joints or Expansion Joints.  We will also discuss one other joint you may see which is typically found between your garage floor and your driveway.  It is called a Construction Joint.  Let me explain the purpose of these joints.

Concrete takes 28 days to cure.  While concrete is curing, moisture is continually escaping from the newly poured concrete.  It escapes up through evaporation, and drains down through the concrete slab.  As the moisture leaves the slab the concrete contracts at a rate of approximately 1/16 of an inch per every 10 feet of concrete.  Concrete is not flexible so as the concrete contracts it will break at the weakest point.  In severe cases it will spider web as the newly poured concrete “contracts” or shrinks as it dries and cures.  Those uncontrolled cracks can be a real eye sore so by cutting a Contraction Joint it creates the weak point and predetermines where that concrete will crack.  If there is no relief point (no Contraction Joints cut) the concrete will crack at random wherever the weakest points are.

Contraction Joints may also be referred to as Control Joints

These joints are cut into slab-on-ground concrete a couple days after the concrete is poured.  They are cut with a Tuckpoint Grinder at a preferred depth of 1/4 of the thickness of the concrete.  As an example a 4 inch residential slab should have 1 inch deep Contraction Joints and that depth is important.  We find that slabs with Contraction Joints cut to specs exhibit much less if any additional random cracking throughout the slab.  The more shallow the cut the more those joints are accompanied by additional cracking or even spiderweb cracking throughout the slab.

It’s important to wait 28 days for new concrete to cure before an epoxy topcoat can be applied.  If you don’t, you’ll have two potential problems.  First the moisture content will be too high and the epoxy to concrete bond will fail.  Second, the shrinking of the concrete will cause the bond to break.  By the time we apply epoxy to your garage floor those Contraction Joints AKA Control Joints have functioned as designed and can be filled in.  Keep in mind we are filling them for appearance purposes we are not gluing the floor back together, that would be virtually impossible.  

Expansion Joints

Are used to isolate a concrete slabs from a neighboring concrete slab or concrete slabs from adjoining concrete structures or walls.  Often confused with Control Joints the primary purpose of the Expansion Joint is to allow independent movement between neighboring concrete structures while hopefully preventing cracking when such movement occurs.

Third type of joint called Construction Joints

Construction Joints are joints designed to keep concrete in place.  Concrete Joints are often starting and stopping points in concrete placed when you don’t pour the entire slab or driveway all at the same time.  These joints are formed using bulkhead which may be plastic or steel but is most often wood.  Many bulkheads are made of wood because they were originally the frame rails or screed rails of the concrete forms.  This is why you frequently find a piece of wood between your driveway and garage floor.  The garage was poured, the frame rail is left at the edge of the garage, and then the driveway is poured.  There’s no code violation and probably no problem until that wood rots away and it will rot away.  You’re then left with a large gap or weeds growing between your driveway and garage.  If necessary we can repair those rotten or unsightly Construction Joints and they’ll even look better than new.

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